1. Field of the Invention
The present invention relates generally to a screen printing method and apparatus to be used in an electronic components mounting process, and particularly to a screen printing method and apparatus of high efficiency in exchanging screens in an automatic manner to increase the productivity and speed of production.
2. Description of Related Art
FIGS. 9-12 show a method of replacing a screen with another in a conventional screen printing apparatus.
FIG. 9 illustrates parts of a screen printing apparatus for exchanging screens and a configuration of a screen exchanger. The numerals 21 and 22 represent the printing apparatus and the automatic screen exchanger, respectively. A screen 23 is fixed in place and released therefrom by a screen locking/unlocking device 25 in the printing apparatus 21. The screen 23 has a printing pattern 23b in which a plurality of screen apertures 23a are formed as shown in FIG. 10. Around the screen 23 is a screen frame 24, which is provided for locking and unlocking the screen 23 by the screen locking/unlocking device 25.
The automatic screen exchanger 22 comprises a stocker unit 26 where the screens 23 are stored, and a lifter unit 27 for lifting and positioning the stocker unit 26 to store a screen 23 in a predetermined stocker unit 26 and to load a desired screen 23 onto the screen printing apparatus 21. A screen transfer unit 28 is provided for transferring the screen 23 between the automatic screen exchanger 22 and the screen printing apparatus 21. A printed circuit board substrate 30 is inserted beneath the screen 23 at the printing position.
FIG. 11 shows a cleaning unit 29 in the screen printing apparatus 21. The cleaning unit 29 is located beneath the screen 23 secured by the screen locking/unlocking device 25 for removing remainings of solder paste from the screen apertures 23a by running forward and backward along the back side of the screen 23.
FIG. 12 is a flowchart for exchanging the screens by the automatic screen exchanger 22 in a prior art arrangement. When the production of print boards using a screen currently selected is completed at Step #101, an NC program is selected for determining a next screen type to be used at Step #102. This is followed by Step #103 where the screen 23 is unlocked at the screen locking/unlocking device 25 for removal. At Step #104, the lifter unit 27 of the automatic screen exchanger 22 is driven upward or downward to position the stocker unit 26 to receive the screen 23. The screen 23 is then transferred and stored in the stocker unit 26 at Step #105. Step #106 follows where the lifter unit 27 is driven upward and downward to position another stocker unit 26 accommodating a desired screen 23 determined by the NC program. The new screen 23 is then transferred from the screen exchanger 22 to the printing apparatus 21 at Step #107 and locked at the locking/unlocking device 25 at Step #108 before restarting the printing operation using the next screen type at Step #109.
In the prior arts, it is necessary to provide such an automatic screen exchanger 22 as described above in addition to the screen printing apparatus 21 for exchanging the screens automatically, which can cause high cost and can require a considerable area for installation. Also, extra space is needed for storage of the screens 23.
The screen apertures 23a shown in FIG. 10 can become prone to be fouled and clogged with the remainings of excess solder paste if the number of printings exceed a certain limit, thereby degrading the quality of the prints, and making it necessary to clean the screen apertures 23a by a cleaning mechanism shown in FIG. 11. When the screen 23 is further used for an additional number of printings or when it has passed a certain duration of time, the screen 23 itself has to be cleaned or replaced with a new one. This can create an extra amount of loss time in the production process.
Thus, the prior art is still seeking a solution to the problem associated with increasing a rate of production and to lower labor costs.